Para-xylylene polymers are employed as coatings for various electronic components due to their desirable physical and electrical properties. One advantage of poly-para-xylylene coatings is that thin layers of such coatings are capable of exhibiting highly desirable physical and electrical properties. Because para-xylylene coatings are applied in very thin layers, heat tends to dissipate rapidly from the underlying components. Thus, the coated components cool down quickly and are less prone to temperature related degradation than similar components bearing other types of coatings.
In further contrast to conventional polymer coatings, para-xylylenes are generally not prepolymerized prior to application on the coatable substrates. This is because the para-xylylene polymers are not given to simple extrusion, melting or molding as are many of the conventional thermoplastics. Additionally, because the para-xylylenes are generally insoluble in commonly used organic solvents, it is impractical to employ traditional solvent deposition techniques for applying poly-para-xylylene coatings.
Accordingly, in most commercial applications, para-xylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the "parylene process." Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400.degree. to 750.degree. C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film.
Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately -70.degree. C.
The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections.
A primary consideration in the parylene deposition process is the achievement of uniform coating thickness on the desired substrates. Unlike conventional polymer coating systems, the condensation deposition of parylene coatings is capable of depositing even ultra-thin films without running or uneven areas resulting upon the substrates, provided that the monomer vapor is homogeneously and evenly distributed on the surface of the substrate. Thus, the design and functioning of the deposition chamber is critical to the achievement of uniform vapor distribution with resultant even coating deposition. Another important consideration in the parylene deposition process is the minimization of waste. Because of the high costs associated with parylene raw materials, there exists substantial economic motivation to preserve and conserve the parylene materials during the coating process.
The parylene deposition process is conducted most efficiently when a relatively large number of substrates are simultaneously coated. However, parylene deposition chambers employed in the prior art are generally deficient in that they provide less than optimal coating uniformity due to an inferior distribution of substrates within the deposition chamber. In this respect, when large numbers of substrates are disposed within the deposition chamber, certain ones of the substrates often contact each other and/or portions of the inner wall of the deposition chamber. Since the substrates in the prior art deposition chambers are not tumbled or otherwise shifted during the deposition process, the coating is not applied to those surfaces of the substrates which are in contact with another substrate or the deposition chamber wall. The present invention overcomes this deficiency associated with prior art deposition chambers by providing a deposition chamber including an internal eccentric tumbler for facilitating the uniform and complete deposition of monomer vapor upon the substrates therewithin.